On Friday, President Joe Biden sent a letter to Congress stating that U.S. forces conducted a military strike in the preceding 48 hours targeting “infrastructure in eastern Syria used by Iran-supported non-state militia groups.” The following day, U.S. Ambassador to the United Nations Linda Thomas-Greenfield submitted a letter to the U.N. Security Council claiming that the United States acted in self-defense consistent with the U.N. Charter.

Several legal experts have raised questions about the legal basis of the strike under international law. In this article, I focus most of my attention on a core part of those concerns. My goal is not to defend the strike, but only to suggest that the right of self-defense under international law—when it comes to use of force in response to a series of attacks—is, at a minimum, less clear-cut than some critics have suggested. Unless we examine the specific legal justification in its proper light, critics and other analysts will risk oversimplifying the issue and falling short.

I should emphasize at the outset that I bracket other issues that would be necessary to resolve in assessing the legality of the U.S. strike on Feb. 25. These issues include the likelihood that the U.S. strike will in fact prevent or deter future attacks, the lawfulness of targeting a non-State armed group when a territorial State is “unwilling or unable” to suppress the threat from that group, the lawfulness of deciding to strike the group inside Syria rather than inside Iraq, and the United States’ peculiar definition of an “armed attack” as indistinguishable from any use of force (an esoteric but important variable in the equation).

But before turning to the core of my analysis, I make four brief observations about international and domestic law related to the Feb. 25 strike.

Four Brief Observations

As a matter of international law, two factors may distinguish the Feb. 25 strike and the United States’ targeted killing of Iranian General Qassem Soleimani in January 2020:

1. Proportionality

The Soleimani strike appeared to violate the condition under international law that military force taken in self-defense must be proportional. As retired general David H. Petraeus wrote at the time, Soleimani “was, in US terms, a combination of CIA Director, JSCO Commander, and Special Presidential Envoy for the Mideast. He was the second most important person in Iran.” It was accordingly difficult to reconcile the targeted killing of this top official with the proportionality rule.

The Feb. 25 strike does not raise those same concerns. Still, it should be noted that White House Press Secretary Jen Psaki and a National Security Council spokesperson stated that the Feb. 25 strike was “proportionate to the prior attacks.” But that’s not how proportionality works. As the International Law Association explained in a major report in 2018:

It might be said that the measures taken in self-defense must be proportionate to the armed attack which preceded them. However, such a pure “equivalence of scale” approach to proportionality is incorrect: the preferred position is that the measures be balanced in light of the aims of the self-defense. As examined above, the legitimate aims in this context are to halt any ongoing attack and prevent the continuation of further attacks.

President Biden’s letter to Congress and Ambassador Thomas-Greenfield’s letter to the United Nations did not repeat the mistaken line about proportionality.

Congress should ask the Biden administration about its conception of proportionality. The earlier formulation (“proportionate to the prior attacks”) sounds more like retaliation than self-defense.

2. Attribution

The Soleimani strike took the fight directly to Iran, and as such relied on a weak, if not invalid, underlying claim that the actions of Iranian-backed militia groups were legally attributable to Iran. The Feb. 25 strike was against the groups themselves, not the government of Iran’s forces, and thus did not raise this question of attribution.

That said, the Feb. 25 strike does raise an issue of attribution that connects the militia groups to the Iraqi – not Iranian – government. Both President Biden’s letter to Congress and Ambassador Thomas-Greenfield’s letter to the United Nations stated that U.S. forces struck the facilities of “Iran-supported non-state militia groups.” But are those groups, in fact, “non-state” actors? The Department of Defense acknowledged that the operation was directed at “a number of Iranian-backed militant groups, including Kait’ib Hezbollah (KH) and Kait’ib Sayyid al-Shuhada (KSS).” What’s important to note is that those groups have been formally incorporated into the Iraqi armed forces (see two articles by Crispin Smith at Just Security on this topic).

Congress should ask the Biden administration how it assesses the status of these militia groups, and what diplomatic, intelligence, and military efforts have been taken to work with the Iraqi government on these matters. In addition to policy concerns, the answers to those questions have important implications under international law (e.g., the condition of necessity in using force against units in another State’s armed forces) and under domestic law (e.g., the strike potentially placing U.S. forces at greater risk in Iraq with no congressional authorization).

As a matter of domestic law, two important factors may distinguish the Feb. 25 strike and the Soleimani strike:

1. Congressional authorizations for use of military force versus Article II

The Trump administration claimed that Congress authorized the lethal operation against Soleimani under the 2002 authorization for the use of military force against Iraq (AUMF) — an extraordinarily farfetched legal claim. In contrast, the Biden administration did not claim to act under any congressional authorization for the use of force; the administration instead claims to have acted pursuant to the president’s authority under Article II of the Constitution. It is not clear whether the location of the Feb. 25 strike – outside of Iraq’s territory – contributed to the decision not to invoke the 2002 AUMF.

Congress should ask the Biden administration whether the location of the Feb. 25 strike affected the administration’s decision not to claim that the 2002 AUMF applied.

As an aside: Over three years ago, I wrote that pressure to forego claims that existing AUMFs cover these kinds of operations can direct pressure toward instead invoking broader conceptions of the president’s constitutional authority under Article II. The same basically holds true today: “If the set of choices is so limited, which stretchiness would you prefer? One that stretches the statutory interpretation under the … AUMF or one that stretches the interpretation of the President’s stand-alone constitutional authority to act without congressional approval?”

2. Risk of escalation

The risk of escalation to warfare with Iran as a result of the Soleimani strike was obviously much greater than the risk of escalation to warfare with Iran or Syria in the Feb. 25 strike.

That distinction might make all the difference in discussions about whether Biden should have obtained congressional approval before the strike on Feb 25. According to long-standing opinions by the Office of Legal Counsel, a key variable in the equation of whether a president must obtain prior congressional authorization before a military operation is the risk of escalation to war. As a Top Expert Backgrounder on military action against Iran explained: “Even in the OLC’s view, the threshold for ‘war’ in the constitutional sense is more easily met when the use of force at issue is against another nation state (rather than in its territory but with its consent) where there is a high likelihood of escalation.” The Soleimani strike was directed against Iran’s senior military official in a way that surely had a substantial likelihood of escalation. So what was the escalatory risk with the Feb. 25 strike?

Congress should ask the Biden administration about its analysis of the risk of escalation. The Defense Department has stated that the Feb. 25 strike was conducted in a manner that “aims to de-escalate the overall situation in both eastern Syria and Iraq.” Even assuming that were true, if the strike had a significant likelihood of escalating to war, this was no decision for one person to make. It would require going to Congress time permitting. Remarkably, the Feb. 25 strike occurred in the territory of one State without its consent (Syria), against groups that are militarily backed by another hostile State (Iran) which have also been formally incorporated as units within the armed forces of a third State (Iraq). Any analysis of risk of escalation must take those factors into account.

International Law on Self-Defense and Recurring Attacks

Imagine a State is subject to a series of armed attacks from an organized armed group (each attack occurs a few days or weeks apart) and the State has high confidence that a proportionate military strike against the group will stop the continuation of these attacks. Does international law permit the State to take that action?

It may be correct that international law would not allow a State to use force in self-defense to respond to an attack that is “clearly over and not ongoing,” but that does not sufficiently address our question. What’s also not exactly on point is the claim that a State cannot use force in self-defense to deter attacks that have not occurred. In our hypothetical case, the State has already been subject to armed attack and is in the midst of being subject to a recurring series of these attacks. Is international law so clear that use of force to deter such armed attacks is prohibited?

In his highly regarded book on the use of force in international law, Tomas Ruys, citing a report by Roberto Ago, Special Rapporteur for the United Nations International Law Commission, writes:

According to Ago: “if [however] the attack in question consisted of a number of successive acts, the requirement of the immediacy of the self-defensive action would have to be looked at in the light of those acts as a whole.” What Ago is actually defending here is the view that if there has been a series of attacks and there is convincing proof that further attacks will follow, the attacked State is not obliged to confine itself to an on-the-spot reaction to the armed attack, but may also exercise its right to self-defense after the latest attack has factually ended. In such situations it is accepted that measures of self-defense may also serve to impede further attacks.

Ruys continues:

Customary practice is replete with examples where the attacked State has (partially) justified its actions by relying on the need to prevent further attacks – i.e., a scenario not to be confused with the pure pre-emptive or preventive model, where no prior armed attack has occurred whatsoever.

For more on the distinction between self-defense after an initial armed attack has occurred versus a pre-emptive model, read Marty Lederman’s analysis of the Obama administration’s position in a speech given by then-State Department Legal Adviser Brian Egan.

The United States has long taken the position that the right of self-defense permits a State to use of force to stop further attacks after an initial attack has occurred. Consider three examples.

Example 1

In 1998, following attacks on US embassies in Kenya and Tanzania, the Clinton administration conducted strikes in Afghanistan and Sudan. The U.S. Ambassador to the United Nations Bill Richardson stated in a letter to the Security Council that the U.S. strike was taken “in response to these terrorist attacks, and to prevent and deter their continuation.”

What was the reaction by other States? Ruys writes: “A number of Western States, such as the UK, Israel, Australia, Germany, France and Spain, approved the US actions without elaborating on their legal basis, or simply expressed understanding. On the other hand, several other States, including Iran, Iraq, Libya and Russia condemned the strikes. Interestingly, both the Non-Aligned Movement and the League of Arab States denounced the strike against the El-Shifa pharmaceutical plant in Khartoum as an act of aggression, but offered no comment on the strikes against Afghanistan. The main criticism undoubtedly related to the lack of reliable intelligence vis-à-vis the target in Khartoum.”

Example 2

In 1993, following a failed assassination attempt against then-former President George H.W. Bush, the Clinton administration launched a strike against Iraqi intelligence headquarters. The U.S. Ambassador to the United Nations Madeleine Albright said in a statement before the Security Council that the U.S. strike was “designed to damage the terrorist infrastructure of the Iraqi regime, reduce its ability to promote terrorism and deter further acts of aggression against the United States.”

What was the reaction by other States? Ruys writes: “Several Council Members quoted with approval the US justification that the action was designed to prevent and deter further attacks.” “The US self-defense claim gained wide support among Council members,” he adds.

Example 3

In 2001, following the attacks on September 11, the George W. Bush administration launched a military intervention in Afghanistan. The U.S. Ambassador to the United Nations John Negroponte stated in a letter to the Security Council that the action was “designed to prevent and deter further attacks on the United States.”

What was the reaction by other States? Ruys writes: “No country seriously contended that the military response was not necessary since the 9/11 attacks were factually ‘over.’ To the contrary, practically the entire international community endorsed the recourse to self-defense.”

So, how closely do the actual events and U.S. assessment of the risks prior to the Feb. 25 strike match our hypothetical and these prior episodes? The stated justifications by U.S. officials follow a similar line. Ambassador Thomas-Greenfield’s letter states that “U.S. and Coalition partner forces in Iraq have been the target of an escalating series of threats and attacks” by the Iran-supported militia groups, and that these groups “are engaged in ongoing planning for future attacks.” The letter also states that the Feb. 25 strike was taken “to defend U.S. personnel and to deter further attacks.” The day before, the National Security Council spokesperson said, “The targets were chosen to correspond to the recent attacks … and to deter the risk of additional attacks over the coming weeks.”

I do not reach a final analysis here on whether international law permits the use of force in the situation described in the hypothetical or in the U.S. presentation of the case. Another leading book on the use of force under international law by Christine Gray, for example, is more equivocal on the international response to the U.S. actions in the 1993 case. And, on my reading of the 1993 Security Council proceedings, Ruys mistakenly discounts statements by the Non-Aligned Movement of States. What’s more, in summarizing the customary international law in this space, Ruys writes:

In all, customary practice indicates that if a State has been subject not to an isolated attack, but to a series of armed attacks, and if there is a considerable likelihood that more attacks will imminently follow, then self-defence is not automatically excluded. (emphasis added)

That formulation is helpful but also unsatisfying. If future armed attacks are imminent, then what work do the prior attacks do? Is it that the State has greater license to use force against those prior attacks (which is, indeed, the issue of “immediacy” and timeliness that Ruys is discussing in this section of his book)? Or is Ruys’ notion of imminence a broad one? Ruys’ summary is, indeed, puzzling because the use of the term “imminently” in his statement is difficult to square with the threats posed by Al Qaeda in October 2001 or by Iraq after the failed assassination attempt months before the U.S. strike.

* * *

In the final analysis, I hope to have provided some analytic tools, albeit incomplete, for assessing the legal justifications for the Feb. 25 strike offered by the United States. As mentioned at the outset, I have left several legal questions out of this analysis. I expect other commentators will pursue those lines of inquiry, and I may join them as well in future.


Editor’s note: Readers may also be interested in Adil Haque’s Biden’s First Strike and the International Law of Self-Defense